efficient permutations in Kotlin

permutations.kt

import org.pcollections.PSet

fun <T> PSet<T>.permutations(): Sequence<Sequence<T>> = when {
    size < 2 -> sequenceOf(asSequence())
    else -> asSequence().flatMap { x -> minus(x).permutations().asSequence().map { sequenceOf(x) + it } }
}